Allow using specific object ID on diff

.changelog/11400.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+cli: Improve `nomad job plan` output for `artifact` and `template` changes
+```

nomad/structs/diff.go

@@ -10,6 +10,14 @@ import (
 	"github.com/mitchellh/hashstructure"
 )
 
+// DiffableWithID defines an object that have an unique and stable field that
+// can be used as an identifier when generating a diff.
+type DiffableWithID interface {
+	// DiffID returns the value to use to match entities between the old and
+	// the new input.
+	DiffID() string
+}
+
 // DiffType denotes the type of a diff object.
 type DiffType string
 
@@ -2419,11 +2427,20 @@ func primitiveObjectSetDiff(old, new []interface{}, filter []string, name string
 	makeSet := func(objects []interface{}) map[string]interface{} {
 		objMap := make(map[string]interface{}, len(objects))
 		for _, obj := range objects {
-			hash, err := hashstructure.Hash(obj, nil)
-			if err != nil {
-				panic(err)
+			var key string
+
+			if diffable, ok := obj.(DiffableWithID); ok {
+				key = diffable.DiffID()
 			}
-			objMap[fmt.Sprintf("%d", hash)] = obj
+
+			if key == "" {
+				hash, err := hashstructure.Hash(obj, nil)
+				if err != nil {
+					panic(err)
+				}
+				key = fmt.Sprintf("%d", hash)
+			}
+			objMap[key] = obj
 		}
 
 		return objMap
@@ -2433,10 +2450,11 @@ func primitiveObjectSetDiff(old, new []interface{}, filter []string, name string
 	newSet := makeSet(new)
 
 	var diffs []*ObjectDiff
-	for k, v := range oldSet {
-		// Deleted
-		if _, ok := newSet[k]; !ok {
-			diffs = append(diffs, primitiveObjectDiff(v, nil, filter, name, contextual))
+	for k, oldObj := range oldSet {
+		newObj := newSet[k]
+		diff := primitiveObjectDiff(oldObj, newObj, filter, name, contextual)
+		if diff != nil {
+			diffs = append(diffs, diff)
 		}
 	}
 	for k, v := range newSet {

nomad/structs/diff_test.go

@@ -4459,7 +4459,7 @@ func TestTaskDiff(t *testing.T) {
 			New: &Task{
 				Artifacts: []*TaskArtifact{
 					{
-						GetterSource: "foo",
+						GetterSource: "foo/bar",
 						GetterOptions: map[string]string{
 							"foo": "bar",
 						},
@@ -4482,6 +4482,18 @@ func TestTaskDiff(t *testing.T) {
 			Expected: &TaskDiff{
 				Type: DiffTypeEdited,
 				Objects: []*ObjectDiff{
+					{
+						Type: DiffTypeEdited,
+						Name: "Artifact",
+						Fields: []*FieldDiff{
+							{
+								Type: DiffTypeEdited,
+								Name: "GetterSource",
+								Old:  "foo",
+								New:  "foo/bar",
+							},
+						},
+					},
 					{
 						Type: DiffTypeAdded,
 						Name: "Artifact",
@@ -6914,7 +6926,7 @@ func TestTaskDiff(t *testing.T) {
 					{
 						SourcePath:   "foo",
 						DestPath:     "bar",
-						EmbeddedTmpl: "baz",
+						EmbeddedTmpl: "baz new",
 						ChangeMode:   "bam",
 						ChangeSignal: "SIGHUP",
 						Splay:        1,
@@ -6934,6 +6946,18 @@ func TestTaskDiff(t *testing.T) {
 			Expected: &TaskDiff{
 				Type: DiffTypeEdited,
 				Objects: []*ObjectDiff{
+					{
+						Type: DiffTypeEdited,
+						Name: "Template",
+						Fields: []*FieldDiff{
+							{
+								Type: DiffTypeEdited,
+								Name: "EmbeddedTmpl",
+								Old:  "baz",
+								New:  "baz new",
+							},
+						},
+					},
 					{
 						Type: DiffTypeAdded,
 						Name: "Template",

nomad/structs/structs.go

@@ -7479,6 +7479,11 @@ func (t *Template) Warnings() error {
 	return mErr.ErrorOrNil()
 }
 
+// DiffID fulfills the DiffableWithID interface.
+func (t *Template) DiffID() string {
+	return t.DestPath
+}
+
 // AllocState records a single event that changes the state of the whole allocation
 type AllocStateField uint8
 
@@ -8120,6 +8125,11 @@ func (ta *TaskArtifact) GoString() string {
 	return fmt.Sprintf("%+v", ta)
 }
 
+// DiffID fulfills the DiffableWithID interface.
+func (ta *TaskArtifact) DiffID() string {
+	return ta.RelativeDest
+}
+
 // hashStringMap appends a deterministic hash of m onto h.
 func hashStringMap(h hash.Hash, m map[string]string) {
 	keys := make([]string, 0, len(m))